1. Field of the Invention
The present invention relates to a circuit board for an ink jet head that ejects ink for printing, a method of manufacturing the circuit board, and an ink jet head using the circuit board.
2. Description of the Related Art
An ink jet printing system has an advantage of low running cost because an ink jet head as a printing means can easily be reduced in size, print a high-resolution image at high speed and even form an image on so-called plain paper that is not given any particular treatment. Other advantages include low noise that is achieved by a non-impact printing system employed by the print head and an ability of the print head to easily perform color printing using multiple color inks.
There are a variety of ejection methods available for the ink jet head to realize the ink jet printing system. Among others, ink jet heads using thermal energy to eject ink, such as those disclosed in U.S. Pat. Nos. 4,723,129 and 4,740,796, generally have a construction in which a plurality of heaters to heat ink to generate a bubble in ink and wires for heater electrical connection are formed in one and the same substrate to fabricate an ink jet head circuit board and in which ink ejection nozzles are formed in the circuit board over their associated heaters. This construction allows for easy and high-precision manufacture, through a process similar to a semiconductor fabrication process, of an ink jet head circuit board incorporating a large number of heaters and wires at high density. This helps to realize higher print resolution and faster printing speed, which in turn contributes to a further reduction in size of the ink jet head and a printing apparatus using it.
FIG. 1 and FIG. 2 are a schematic plan view of a heater in a general ink jet head circuit board and a cross-sectional view taken along the line II-II of FIG. 1. As shown in FIG. 2, on a substrate 120 is formed a resistor layer 107 as a lower layer, over which an electrode wire layer 103 is formed as an upper layer. A part of the electrode wire layer 103 is removed to expose the resistor layer 107 to form a heater 102. Electrode wire patterns 205, 207 are wired on the substrate 120 and connected to a drive element circuit and external power supply terminals for supply of electricity from outside. The resistor layer 107 is formed of a material with high electric resistance. Supplying an electric current from outside to the electrode wire layer 103 causes the heater 102, a portion where no electrode wire layer 103 exists, to generate heat energy creating a bubble in ink. Materials of the electrode wire layer 103 mainly include aluminum or aluminum alloy.
In such an ink jet head circuit board, the heater 102 is subjected to a severe environment, including a temperature rise and fall as large as 1,000° C. in a short period of time and also mechanical impacts caused by cavitations from repeated creation and collapse of bubbles. To deal with this situation, the heater 102 is insulated and protected from ink by multiple protective layers, which comprise a protective insulation layer 108 of inorganic compounds, such as SiO and SiN, and a metal layer 110 deposited over the insulation layer 108 which is made from a mechanically more stable metal, such as Ta (this layer may also be called an anticavitation layer because of its capability of withstanding damages from cavitations) (see FIG. 2). In addition, the similar construction is also formed over the electrode wire layer 103—which provides electrical connection for the resistor layer 107—to prevent corrosion by ink.
In ink jet printers, there are growing demands in recent years that they have a capability of printing images of high resolution and quality at high speed. This requires a large number of ink ejection nozzles and energy generation elements, such as heaters used to eject ink, to be formed in a substrate at high density. In arranging a large number of nozzles and energy generation elements in the substrate at high density, a reduction in power consumption by the energy generation elements is particularly important.
An example construction capable of reducing power consumption by the energy generation elements is disclosed in Japanese Patent No. 3382424.
FIG. 3 shows a schematic cross-section of a heater in an ink jet head circuit board disclosed in Japanese Patent No. 3382424, the cross-sectioned portion corresponding in position to the line II-II of FIG. 1. In this construction, first and second protective insulation layers 108a, 108b are formed over the electrode wire layer 103, with the lower layer or the first protective insulation layer 108a removed from above the heater 102. That is, Japanese Patent No. 3382424 discloses a construction in which an overall thickness of the protective layer over the heater is made smaller than that over the electrode wire. This construction improves an energy efficiency by reducing the effective thickness of the protective layer over the heater 102 and at the same time provides a required protective insulation function by the second protective insulation layer 108b. This construction therefore can achieve a reduction in power consumption by the heater without degrading the protective performance of the protective layer.
In addition to improving the thermal efficiency of the heaters, it is also important to reduce resistances of electrode wires from the standpoint of reducing an overall power consumption of the circuit board. Normally, a reduction in resistance of the electrode wires is achieved by increasing the width of the electrode wires formed on the board. However, as the number of heaters or energy generation portions formed on the board becomes very large for the reason described above, a sufficient space to accommodate widened electrode wires cannot be secured without increasing the size of the circuit board.
In this circumstance, the inventors of this invention studied the possibility of reducing the electrode wire resistance by increasing the thickness of the electrode wires. Having built a construction in which the electrode wires are increased in thickness and in which the total thickness of the protective layers over the heaters is made smaller than the total thickness of the protective layers over the electrode wires, as shown in FIG. 3, the inventors of this invention have found a new problem as described below.
Considering the coverage over the stepped portions of the electrode wires bordering the heaters, the protective layers need to be increased in thickness as the electrode wire thickness becomes large. This prevents the protective layers over the heaters from being formed sufficiently thin or results in an increase in a space or area accommodating the thick portion of the protective layers over the heaters. As a result, the advantage of a reduced power consumption of the heaters brought about by the above construction is offset by these disadvantages.